With the discovery of complex metal oxides that are superconductors at relatively high temperatures compared with traditional superconducting materials, there has been renewed interest in operating cryogenic refrigerators at about 70.degree. K. Although this temperature is not particularly difficult to achieve, being only a few degrees lower than that at which liquid nitrogen boils under a pressure of one atmosphere, it has hitherto made necessary the use of vacuum insulation to maintain a reasonable standard of thermal insulation about the chamber or other vessel in which the superconductor operates. Whereas, for example, pipelines that conduct liquid nitrogen from a storage vessel to a plant in which it is used, may be vacuum-insulated, in practice, alternative thermal insulation typically comprising a mineral powder such as perlite is used and is housed in a space defined between the pipeline and a sleeve surrounding the pipeline. Since air will condense on one side of the cold wall the other side of which is in contact with liquid nitrogen, thereby causing a hazard and also reducing the effectiveness of insulation, it is conventional to purge the space in which the mineral powder insulant is housed with dry nitrogen. However, if the thermal insulation is intended to maintain operating temperatures below that of liquid nitrogen, the technique of nitrogen purging the insulant space is not effective as the nitrogen will itself come into contact with the cold wall and will condense.
UK Patent specification 1 425 614 discloses a cryogenic insulating sleeve for use in systems where fluids flow through transmission pipelines at cryogenic temperatures wherein the sleeve is of the vacuum type, and comprises a perlitic tubular core within a sealed envelope which envelope contains a condensible gas which fills the envelope at normal temperature and pressure conditions and which condenses a cryogenic temperature substantially to evacuate the envelope. This is an alternative kind of thermal insulation for use in thermally insulating a cryogenic pipeline conducting liquefied natural gas. It does not address the problem of providing thermal insulation for apparatus at a temperature below 77.degree. K.